IMAGE ACQUISITION GUIDELINES FOR SFM

IMAGE ACQUISITION GUIDELINES FOR SFM a.k.a. Close-range photogrammetry (as opposed to aerial/satellite photogrammetry) Basic SfM requirements (The Golden Rule): minimum of 60% overlap between the adjacent photographs, more is preferred each point should be visible/recognizable in at least 3 photographs

SHOOTING (COVERAGE) STRATEGIES think aerial photography: even coverage (image mosaicking) following the plane-of-interest

SHOOTING (COVERAGE) STRATEGIES move along the outcrop and avoid shooting from a single position (N.B.: but do take photos from various angles to improve 3D coverage!) keeping the same distance from the object keeps the scale of the object constant it might actually be a good idea to pre-plan your shooting session

SHOOTING PLATFORMS your feet (obviously)

SHOOTING PLATFORMS elevated platforms may be required to obtain better coverage/perspective remote triggering of the camera may be neccesary

SHOOTING PLATFORMS Unmanned Aerial Vehicles (UAV s), in order of sophistication: tethered kites and helium baloons or blimps

SHOOTING PLATFORMS Unmanned Aerial Vehicles (UAV s), in order of sophistication: remotely controlled or autonomous multicopters and airplanes ( drones )

SHOOTING PLATFORMS real aircraft for hire: hot-air baloons, ultralight planes, gyrocopters, helicopters... (alternatively: obtain/purchase existing aerial imagery)

CAMERA/LENS CONSIDERATIONS reasonably high image resolution (not too large either) manual control RAW/TIFF image format preferred to JPEG image noise: avoid high ISO values; larger sensor is better weight/compactness (particularly for remote operation)

CAMERA/LENS CONSIDERATIONS CONTINUED lens quality is important (particularly in the image corners) quality-wise, fixed focals are generally better than zooms use optimum aperture values for maximum image sharpness and depth-of-field per-pixel view (100% zoom) extreme image corner per-pixel view (100% zoom) near-center

CAMERA/LENS CONSIDERATIONS CONTINUED lens quality is important (particularly in the image corners) quality-wise, fixed focals are generally better than zooms use optimum aperture values for maximum image sharpness and depth-of-field Image sharpness ( blur index ) is measured accross the frame at various apertures. Smaller blur index values are better. Anything below 2 (purple colors) is optically top-class. data from camera lens testing site SLRgear.com

CAMERA/LENS CONSIDERATIONS CONTINUED avoid ultrawide lenses due to large distortions (N.B.: Photoscan can now process fisheye images) avoid telephoto lens due to lack of 3-dimensionality of the image (flattened field of view) with zooms, do not randomly change the focal length during shooting

SHOOTING CONDITIONS if possible, shoot in optimal lighting conditions (cloudy?) avoid shooting moving objects avoid shooting reflective surfaces avoid unwanted image elements (vegetation, cars,...) can be corrected later with masking use camera tripod and/or optical image stabilization (No. 1 problem in my experience!) OTHER: do not crop or otherwise geometrically modify photographs (but image downsampling is possible and sometimes desirable)

SHOOTING CONDITIONS DYNAMIC RANGE the range of brightness values in the scene may greatly exceed the imaging sensor capabilities for maximum information, combine several exposures into one (High Dynamic Range imaging) HDR may be done: inside the camera when supported (convenient, but restricted and produces JPEG only) in postprocessing (max flexibility, but time-consuming and requires skill and experience) in either case, a tripod is a must

EXPECTED MODEL RESOLUTION W Example: Sony RX100 at 28 mm-e focal length image resolution in pixels 5472 x 3648 (20 Mp) horizontal viewing angle of the lens: = 64.8 distance from camera to object D D field of view in m: W = 2 D tan ( α 2 ) size of 1 pixel in m: W/5472 D (m) W (m) pixel size (cm) Shootpoint separation (m) @ 65% overlap 1 1.27 0.02 0.30 2 2.54 0.05 0.59 3 3.81 0.07 0.89 4 5.08 0.09 1.18 5 6.35 0.12 1.48 6 7.62 0.14 1.78 10 12.69 0.23 2.96 20 25.38 0.46 5.92 30 38.08 0.70 8.88 50 63.46 1.16 14.81 100 126.92 2.32 29.62 150 190.39 3.48 44.42 200 253.85 4.64 59.23 300 380.77 6.96 88.85 500 634.62 11.60 148.08 1000 1269.24 23.20 296.16

EXPECTED MODEL RESOLUTION Example: Sony RX100 at 28 mm-e focal length image resolution in pixels 5472 x 3648 (20 Mp) horizontal viewing angle of the lens: = 64.8 distance from camera to object D field of view in m: W = 2 D tan ( α 2 ) size of 1 pixel in m: W/5472 SEPARATION D (m) W (m) pixel size (cm) Shootpoint separation (m) @ 65% overlap 1 1.27 0.02 0.30 2 2.54 0.05 0.59 3 3.81 0.07 0.89 4 5.08 0.09 1.18 5 6.35 0.12 1.48 6 7.62 0.14 1.78 10 12.69 0.23 2.96 20 25.38 0.46 5.92 30 38.08 0.70 8.88 50 63.46 1.16 14.81 100 126.92 2.32 29.62 150 190.39 3.48 44.42 200 253.85 4.64 59.23 300 380.77 6.96 88.85 500 634.62 11.60 148.08 1000 1269.24 23.20 296.16

SOME CAMERA EXAMPLES Sony RX100 MK II a high-end large-sensor compact camera sensor size 13.2 x 8.8 mm image resolution in pixels 5472 x 3648 (20 Mp) integrated high-quality 28-100 mm-e zoom lens weight: 281 g price: 490 EUR (was ~800 EUR) my pocket field camera of choice

SOME CAMERA EXAMPLES Olympus OM-D E-M1 a top-of-the-line, pro-grade weather-resistant mirrorless camera of the 4/3 system sensor size 17.3 x 13 mm image resolution in pixels 4608 x 3456 (16 Mp) weight: 497 g (879 g with pro zoom lens) price: 1.290 EUR (1.950 EUR with pro zoom lens) for field workers, probably the best compromise in image quality vs. weight&bulk vs. robustness LENSES: 24-80 mm-e f/2.8 50 mm-e f/1.8 90 mm-e f/1.8 150 mm-e f/1.8 (total price for the setup: 3.967 EUR)

SOME CAMERA EXAMPLES Nikon D810 one of the highest resolution and image quality SLR camera there is sensor size 35.9 x 24 mm image resolution in pixels 7360 x 4912 (36 Mp) weight: 980 g (1880 g with pro zoom lens) price: 3.100 EUR (4.660 EUR with pro zoom lens) LENSES: 24-70 mm-e f/2.8 50 mm-e f/1.4 85 mm-e f/1.4 (total price for the setup: 6.324 EUR)

(FYI: THE BEST MONEY CAN BUY) Phase One IQ180 645DF medium format system sensor size 53.7 x 40.4 mm image resolution in pixels 10328 x 7760 (80 Mp) price: ~ 45.000 EUR with 80 f/2.8 lens

MEGAPIXELS... what does it mean in pratice? Nikon D810 + Nikkor AF-S 50 mm/1.4 G image resolution 7360 x 4912 (38 Mp) JPEG size: 29.1 MB RAW size: 50.4 MB Olympus E-M1 + M.Zuiko 25 mm/1.8 image resolution 4608 x 3456 (16 Mp) JPEG size: 7.1 MB RAW size: 16.9 MB

MEGAPIXELS... what does it mean in pratice? 100% zoom 100% zoom Nikon D810 + Nikkor AF-S 50 mm/1.4 G image resolution 7360 x 4912 (38 Mp) JPEG size: 29.1 MB RAW size: 50.4 MB Olympus E-M1 + M.Zuiko 25 mm/1.8 image resolution 4608 x 3456 (16 Mp) JPEG size: 7.1 MB RAW size: 16.9 MB